Issue 8, 2014 - Research on heteropolyacid flue gas desulfurization and denitrification technology

Research on heteropolyacid flue gas desulfurization and denitrification technology

School of Architecture and Environment, Sichuan University, Jiang Dan, National Engineering Research Center for Flue Gas Desulfurization, Li Jianjun

For a long time, China's energy structure has been dominated by coal, and in 2013, China's coal consumption accounted for 65.
7%
of total energy consumption.
A large amount of toxic and harmful substances such as soot, sulfur dioxide, nitrogen oxides and other toxic and harmful substances produced by burning coal have caused serious environmental pollution
.
With the development of China's social economy, the environmental problems faced by people are becoming increasingly severe
.
According to the 2012 Bulletin on China's Environmental Conditions, the ambient air quality of 325 cities at the prefecture level and above still implement the Ambient Air Quality Standard (GB3095-1996), and the proportion of cities that meet this standard is 91.
4%, but after the implementation of the new standard Ambient Air Quality Standard (GB3095-2012), the proportion of cities that meet the standard is only 40.
9%; The proportion of cities that met the ambient air quality standards in the 113 key cities for environmental protection was 88.
5%, and the proportion of cities that met the standards was only 23.
9%
according to the new ambient air quality standards.
The huge coal energy consumption cannot be replaced by other energy sources on a large scale for a while, so the desulfurization and denitrification of coal-fired flue gas is an urgent issue
to be solved.

There are many methods of flue gas desulfurization and denitrification, but each method has its own shortcomings that are difficult to overcome, such as a large amount of solid waste generated by limestone/gypsum flue gas desulfurization is difficult to treat and dispose of, the investment and operation cost of selective catalytic reduction denitration is high, the operating temperature is high, and the catalyst is easy to block and wear, so it is of great significance
to find a more economical and effective desulfurization and denitrification process.
The use of heteropoly acid desulfurization and denitrification is a new hot spot
in current research.
Heteropoly acid is an oxygen-containing poly acid that is connected by heteroatoms and polyatoms through the coordination bridge of oxygen atoms according to a certain structure, which is a class of functional compounds that have attracted much attention in the field of new technologies of green chemistry and chemical industry, and is also a kind of "green catalyst"
.
Heteropoly compounds have the characteristics
of large molecular volume, excellent transport and storage capacity for electrons and protons, high thermal stability, activity of "lattice oxygen", high proton acidity, and non-toxic, odorless, non-volatile and easy to separate.
It has been widely used in the field of catalysis in organic synthesis industry and environmental protection fields, such as catalytic esterification of organic matter, dehydration cyclization reaction, photocatalytic reaction, etc.
, and catalytic oxidative desulfurization of
oils.

1.
Desulfurization and denitrification of heteropoly acid aqueous solution

Many studies have shown that heteropoly acid solution has a good application prospect
of flue gas desulfurization and denitrification.
The use of heteropoly acid to remove SO2 and NOx, low cost, no pollution of the product, can also be used
as fertilizer after recovery.
Heteropoly acid desulfurization and denitrification mainly uses the redox ability
of metal ions.
Zhao Youcai of Tongji University and others studied the desulfurization and denitrification reaction of molybdenum silicic acid, SO2 was first absorbed, and then oxidized by heteropoly acid to form sulfuric acid, and heteropoly acid was reduced.
The reduced heteropoly acid is oxidized to heteropoly acid under the action of oxidants such as NOx, and so that the cycle can form an autocatalytic redox system, so it can also be applied to simultaneous desulfurization and denitrification
.

The influencing factors of desulfurization and denitrification of heteropolyacid solution include absorption solution concentration, temperature, PH value, etc
.
The greater the concentration of heteropoly acid aqueous solution within a certain range, the higher the absorption efficiency, but the concentration of heteropoly acid solution needs to comprehensively consider factors
such as economy and absorption efficiency.
As the temperature increases, the absorption capacity of heteropoly acids decreases
.
Heteropoly acids absorb SO2, first physical absorption, followed by chemical reactions
.
In the process of physical absorption, because SO2 dissolved in water is an exothermic reaction, the temperature rises, the solubility of SO2 in the solution decreases, and in the process of chemical reaction, the temperature increase has a promoting effect on the reaction itself, but the promotion effect can not offset the negative effect of the physical absorption process, thus showing a trend of desulfurization efficiency decreasing with the increase of temperature, so better desulfurization efficiency
can be obtained at lower temperature or room temperature.
SO2 and NOx are acid gases, the larger the pH value of the solution, the greater the desulfurization and denitrification efficiency, but in the actual situation, the corrosion of
the equipment needs to be considered.

In order to seek the desulfurization efficiency of efficient heteropoly acid aqueous solution, Wang Rui et al.
of Hainan University investigated the flue gas desulfurization efficiency of H3PW12O40, H4SiW12O40, H7PMo12O40 and Na2HPMo12O40 aqueous solutions, and the results showed that Na2HPMo12O40 had the highest
desulfurization rate.
At the same time, adding additives (NaCl, NH4VO3, CuSO4, H3PO4, etc.
) to heteropolyacid solution can also improve the desulfurization rate
。 Zhao Ling of Sichuan University and others studied the influence of heteroatoms and polyatoms in heteropoly acid and their proportions on desulfurization efficiency, and found that among the three aqueous solutions of H3PMo12O40, H3PW12O40 and H4SiW12O40, phosphomolybdic acid had the best desulfurization performance.
Doping different proportions of vanadium atoms in phosphomolybdic acid, the desulfurization performance will be improved to a certain extent, and the desulfurization performance order is: H6PMo9V3O40>H5PMo10V2O40>H4PMo11VO40
.
In terms of removing NOx from flue gas using heteropoly acid aqueous solution, Zheng Zuyi of North University of China studied the absorption efficiency of phosphotungstic acid and phosphomolybdic acid on nitrogen oxides, and found that the absorption efficiency of phosphotungstic acid was higher than that of phosphotomolybdic acid under certain conditions, and the absorption efficiency of phosphotomolybdic acid could reach 85.
6%.

2.
Solid-loaded heteropolyacid desulfurization and denitrification

Heteropoly acid aqueous solution has certain defects in the actual application process, its thermal stability is poor, and it is difficult to recover
when dissolved in solution.
The heteropoly acid loaded to the suitable support, can overcome the above defects, due to the increase of the specific surface area, conducive to the reaction, thermal stability is also improved to a certain extent, and the loaded heteropoly acid is not easy to lose, the reactant and the catalyst are easy to separate, regenerated can be reused
.
The solidification of heteropoly acids is conducive to the heterogeneous of catalytic reactions, which is not limited to homogeneous catalysis, improves catalytic activity, and simplifies the production process to make it more widely used
.

Many studies have loaded heteropoly acids on porous solid supports, including metal oxides such as SiO2, TiO2, Al2O3, activated carbon and mesoporous molecular sieve
.
In the early days, Chu Wenling and others from the Chunying Institute of Chinese Sciences studied the adsorption of heteropoly acids on several domestic activated carbons, and the results showed that the adsorption capacity of different heteropoly acids on activated carbon was different, among which coal-based carbon had the highest adsorption capacity; The acidic group on the surface of activated carbon is not conducive to the adsorption of heteropoly acid, and the basic pyrone structure is conducive to the adsorption
of heteropoly acid.
Wu Yue et al.
used porous materials as carriers to investigate the catalytic activity of supported heteropoly acid catalysts, and discussed the nature of the role of the intrinsic properties of the support in the solidification, adsorption and catalytic reactions of heteropoly
acids.

At present, there are many studies on the removal of nitrogen oxides from flue gas by supported heteropoly acids, and most of the methods used in research are adsorption decomposition method, and some scholars have studied catalytic oxidation denitrification, which mainly oxidizes NO in flue gas to NOx, so as to achieve the purpose of
denitrification 。 Wang Qun and others of Ocean University of China loaded phosphotungstic acid on modified activated carbon to investigate the catalytic oxidative denitrification activity of the catalyst, the temperature was 120 °C, the space velocity was 1000h-1, the O2 volume fraction was 8%, the H2O volume fraction was 6%, the NO content was 443mg/m3, and the phosphotungstic acid loading mass fraction was 10%, and the NO removal efficiency reached 62%.


Regarding the mechanism of heteropoly acid adsorption and decomposition of nitrogen oxides, foreign scholar Yang proposed the following reaction, under the condition of low temperature and low NO concentration, the water molecular connection can be easily replaced by the NO connection to form H3PW12O40·3NO, and the NO connection in NO saturated phosphotungstic acid is a protonized NO in the form of ions, that is, (NOH)+
.

Many studies have shown that the catalytic decomposition of NOx by supported heteropoly acid catalysts is higher than that of pure heteropoly acid catalysts
.
Zhang Xueyang et al.
of Shandong University studied and prepared a "ship in bottle" catalyst HPW-NaY containing only phosphotungstic acid, and studied its catalytic decomposition of NOx, the conversion rate of HPW-NaY catalytic decomposition of NOx is 61%, while the conversion rate of HPW catalyst to NOx is 54%, and its catalytic decomposition of NOx is not as good as HPW-NaY
.
Song Shumei et al.
prepared a new polyacid catalytic system with the function of adsorption and decomposition of NOx, and investigated the adsorption and decomposition performance
of the system for NOx.
The results show that tungsten heteropoly acid is superior to molybdenum, and tungsten with phosphomolybdic acid as the parent replaces heteropoly acid H3PMo12-xWxO40 (x=1,3,6,12), and the denitration performance is gradually enhanced
with the increase of the number of tungsten atoms.
The phosphotungstic acid catalyst with titanium dioxide and carbon nanotubes as the support has good adsorption performance for NOx, and its adsorption efficiency for NOx is higher than that of single phosphotungstic acid and single carbon nanotube, and the performance of carbon nanotube support is better than that of titanium dioxide support
.

In addition to loading heteropolycompounds on the support, certain metal elements can also be added to heteropoly acids to support on the support to improve catalyst activity
.
A foreign scholar studied the addition of Pt, Rh and Pd to HPW respectively, and then loaded onto composite metal oxide supports such as Zr-Ce or Zr-Ti, and the catalyst activity was improved to a certain extent, due to the interaction between metal elements and HPW and the promotion of
the carrier itself 。 Hussein Hamad et al.
loaded Pt, HPW and Pt/HPW onto the mesoporous molecular sieve MSU, investigated the influence of various factors on their denitration efficiency, and found that the catalyst containing a small amount of Pt and a certain amount of HPW had high catalytic activity, and the catalytic activity was as high as 97%
at 250 °C and O2 volume fraction of 1%.

3.
Conclusion

Among many flue gas desulfurization and denitrification technologies, heteropoly acid desulfurization and denitrification is currently at the forefront of research topics, and there are still many problems to be studied
.
At present, there are many studies on the adsorption and decomposition of NOx supported heteropoly acid catalysts, but there are fewer studies on the removal of SO2 in flue gas, and it is necessary to study the flue gas desulfurization of supported heteropoly acid catalysts to seek efficient desulfurization catalysts
.
Heteropoly acid has excellent reversible redox performance, and the development and utilization of this performance while desulfurization and denitrification are also the direction of further research.
At present, it is still necessary to understand its complex reaction mechanism and carry out thermodynamic and kinetic studies
.
Heteropoly acid desulfurization and denitrification technology has not yet reached the level of industrial application, and the development of heteropolyacid/carrier composite catalysts with excellent performance and the improvement of desulfurization and denitrification rate are also the focus of
future research.